Artificial diamonds - now available in extra large

Diamonds are a girl's best friend, they say - and soon they could be every girl's best friend.

A team in the US has brought the world one step closer to cheap, mass-produced, perfect diamonds. The improvement also means there is no theoretical limit on the size of diamonds that can be grown in the lab.

A team led by Russell Hemley, of the Carnegie Institute of Washington, makes diamonds by chemical vapour deposition (CVD), where carbon atoms in a gas are deposited on a surface to produce diamond crystals.

The CVD process produces rapid diamond growth, but impurities from the gas are absorbed and the diamonds take on a brownish tint.

These defects can be purged by a costly high-pressure, high-temperature treatment called annealing. However, only relatively small diamonds can be produced this way: the largest so far being a 34-carat yellow diamond about 1 centimetre wide.

Microwaved gems

Now Hemley and his team have got around the size limit by using microwaves to "cook" their diamonds in a hydrogen plasma at 2200 °C but at low pressure. Diamond size is now limited only by the size of the microwave chamber used.

"The most exciting aspect of this new annealing process is the unlimited size of the crystals that can be treated. The breakthrough will allow us to push to kilocarat diamonds of high optical quality," says Hemley's Carnegie Institute colleague Ho-kwang Mao.

"The microwave unit is also significantly less expensive than a large high-pressure apparatus," adds Yufei Meng, who also participated in the experiments.

The new technique is so efficient that the synthetic diamonds contain fewer impurities than those found in nature, says Meng. "We once sent one of our lab-grown diamonds for jewellery identification, it wasn't told apart from natural ones," she says.

One immediate application will be to make ultra-high quality windows that are optically transparent to lasers.

Threat to commerce

The team's method "could be routinely run in any laboratory where it is needed," says Alexandre Zaitsev, a physicist at the City University of New York, whose work also includes diamonds. "When considered in combination with the high-growth-rate technique of CVD diamonds, it seems to be a starting point of mass-scale production of perfect diamond material at a low price."

Zaitsev considers low-pressure annealing at temperatures greater than 2000 °C to be a "breakthrough in diamond research and technology".

The improving quality of synthetic diamonds threatens the natural diamond market. While 20 tonnes of natural diamonds are mined annually, some 600 tonnes of synthetic diamonds are produced each year for industrial use alone.

They are used in a range of high-end technologies, such as lasers and high-pressure anvils. Some companies have also started to sell synthetic diamonds as gemstones. In response, diamond giant De Beers has set up a "Gem Defensive Programme" with the aim of finding ways to tell apart synthetic and natural diamonds.

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The CVD diamond in the centre has not been annealed, the ones to the left and right have (Image: Hemley /PNAS)